This invention relates generally to a process and equipment for producing trichlorosilane (HSiCl.sub.3) and to the use of same for producing silicon, and more particularly, to a two stage trichlorosilane production process and equipment and to their incorporation into a closed loop silicon production process.
Nearly all of the high purity silicon used in manufacturing semiconductor devices is produced by chemical vapor deposition from a mixture of trichlorosilane and hydrogen onto hot filaments in a bell jar type reactor according to the reaction ##STR1##
The production of silicon in this manner requires large quantities of pure trichlorosilane (HSiCl.sub.3), especially in view of the fact that only about 1/3 of the available silicon actually results in silicon deposition. The remaining available silicon is present in the reaction effluent as silicon tetrachloride (SiCl.sub.4) or unreacted trichlorosilane. A molar silicon tetrachloride to hydrogen chloride ratio of approximately 2:1 is typical of the effluent from the hot filament type reactor.
Trichlorosilane for the reaction is usually provided by reacting hydrogen chloride with silicon such as metallurgical grade silicon (an impure form of silicon) according to the reaction ##STR2## Anhydrous hydrogen chloride is injected into a bed of silicon particles in a fluidized bed type of reactor. After purification, the trichlorosilane produced by this reaction is suitable for high purity silicon production.
While the reaction of HCl with silicon to produce trichlorosilane is, by itself, very efficient, the reaction has a number of limitations, especially as applied to a closed loop production system. First, the reaction does not make use of the silicon tetrachloride which is the by-product of the silicon deposition reaction. In the deposition reaction about 70% by weight of the trichlorosilane input is recovered as silicon tetrachloride. A cost efficient process must recycle this silicon tetrachloride to more efficiently utilize this available silicon. Additionally, the production of semiconductor grade trichlorosilane from the HCl reaction requires extremely pure HCl. Most sources of HCl contain trace amounts of organic contaminants. These organic contaminants are carried through the reaction, are incorporated with the trichlorosilane, and subsequently are incorporated into the deposited silicon as an unwanted impurity. Still further, the reaction of HCl with silicon to produce trichlorosilane is an extremely exothermic reaction and requires heat exchange to minimize overheating. Even with heat exchange, however, hot spots do occur which reduce the trichlorosilane conversion efficiency and degrade the reactor.
The recycling of the SiCl.sub.4 by-product is an important consideration in the overall economics of the silicon production process. The by-product SiCl.sub.4 is usually hydrolyzed or burned in an oxygen/hydrogen mixture to recover HCl and by-product SiO.sub.2. The HCl is then useful as an input to the trichlorosilane production reaction. Thus using the SiCl.sub.4, however, is an expensive process and greatly increases the cost of producing HSiCl.sub.3.
Trichlorosilane has also been made by the direct hydrogenation of silicon tetrachloride at high temperatures and pressures according to the reaction EQU 3SiCl.sub.4 +2H.sub.2 +Si.revreaction.4HSiCl.sub.3.
Conversion efficiencies of about 20-23% have been achieved with this process when operated at 650.degree. C. and 345 kPa using a H.sub.2 /SiCl.sub.4 ratio of 2:1, a residence time of 1 sec. and with a 5% CuCl catalyst mixed with the silicon.
The cost of trichlorosilane and thus the cost of silicon produced therefrom is a factor in the production cost of any silicon semiconductor device. It is therefore an object of this invention to provide an improved process for the production of trichlorosilane which is more economically feasible than prior art processes.
It is another object of this invention to provide improved equipment for the production of trichlorosilane.
It is yet another object of this invention to provide an improved process for the production of polycrystalline silicon in the form of a closed loop process which efficiently utilizes input reactant materials.
It is a still another object of this invention to provide a process for the production of a higher purity trichlorosilane.